1. Field of the Invention
This invention relates generally to multiple access communication in digital radio systems, and more particularly to improvements in the multiple access communication from one or more centrally based nodes having multibeam antennas to fixed remote user terminals and/or mobile user terminals.
2. Background
Multiple access radio systems provide communication services for fixed remote user terminals and/or mobile user terminals. Examples of multiple access radio systems include land mobile radio networks, cellular mobile radio networks, and wideband radio networks between one or more central nodes and fixed subscribers. The central node in a multiple access radio system may use multibeam antennas for increasing system capacity and improving communications quality. The forward link or downlink in a multiple access radio system is a communications link between a central node and a fixed remote or mobile user terminal. The central node can be located at either a fixed location on the Earth in a terrestrial radio system or as part of an orbiting satellite in a satellite radio system.
Digital radio systems transmit and receive digital message information, e.g., computer or Internet data. Alternatively, digital radio systems accept analog message information, e.g., voice or video data, and convert this analog information to a digital format during transmission and reception. Accordingly, a central node transmits message information in a digital format using downlink beams defined by a multibeam antenna to a fixed remote or mobile user terminal where the receiver processes the digital message information to extract user message information. In some satellite radio systems, the central node processing is divided between a satellite repeater and a ground-based station processor.
User terminals within the same beam coverage area generally avoid mutual interference through the use of some form of multiple access scheme. Conventional multiple access radio services use Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), or some combination thereof. Generally, FDMA separates users into different frequency subbands; TDMA separates users into different time intervals or slots; and, CDMA separates users by assigning different signature waveforms or codes to each user. These CDMA codes can be either orthogonal, i.e., there is no interference between synchronized users, or quasi-orthogonal, i.e., there is some small interference between users. FDMA and TDMA are orthogonal multiple access (OMA) schemes because with ideal frequency filters and synchronization there is no mutual interference. Another example of an OMA system is CDMA with orthogonal codes. Quasi-Orthogonal Multiple Access (QOMA) systems include CDMA with quasi-orthogonal codes and FDMA/TDMA with randomized frequency hopping.
For an isolated beam, an OMA scheme generally provides a larger system capacity than a QOMA scheme. However, when other beams are taken into account, practical systems often use QOMA schemes for reducing interference between users to acceptable levels.
Interference between a user in one beam and users in other beams is normally reduced by crossbeam antenna attenuation. However, in OMA radio systems, such cross-beam attenuation usually does not reduce interference enough to allow the reuse of the same orthogonal waveform or channel in adjacent beams. Instead, channel management is typically required for determining when a multiple access channel can be reused in another beam. This leads to a reuse factor that is less than 1. The reuse factor for an orthogonal channel is defined as the number of user terminal assignments to that orthogonal channel in different beam coverage regions divided by the total number of beam coverage regions. Because the capacity of a multiple access system is proportional to the average value of the reuse factor with respect to all the multiple access channels, it is desirable to make the reuse factor for each multiple access channel as large as possible subject to interference constraints. Practical limitations on multibeam antennas typically cause the reuse factor in conventional cellular OMA systems to vary between ⅓ and 1/12.
In contrast, in a QOMA radio system, e.g., the uplink of a CDMA radio system in the IS-95 standard, the reuse factor can be unity because the crossbeam antenna attenuation can be sufficient to keep mutual interference between users in different beams to adequately small levels. However, one drawback is that a QOMA radio system generally has a theoretical capacity that is less than that of an OMA radio system.
Conventional multiple access digital radio systems provide means for coding/decoding message information for error correction, means for interleaving/deinterleaving the message information, and a transmission format for the message information that includes a reference signal. The reference signal is generated and transmitted at the central node and used by the user terminal receiver for obtaining channel parameters to aid in demodulating a user signal.
Further, the message information is conventionally coded for transmission on both quadrature axes of a radio frequency carrier, e.g., cos ωt and sin ωt. One example of quadrature coding is to alternate coded symbols between the two axes. Error correction coding techniques that use a complex signal constellation also exploit both quadrature axes. For a single-axis coder in which the coder only exploits one of the two quadrature axes, i.e., the coder output is real, the information rate is reduced by a factor of two relative to quadrature coding. On the other hand this dimension reduction provides a more robust signal form in the presence of interference.
In downlink transmissions from a central node to a particular user terminal, the central node transmitter may include a multibeam antenna and one of these beams includes the particular user terminal. Generally the user terminal has a single antenna for receiving the downlink transmission. Adaptive equalization of multiple antenna signals cannot be applied to a downlink system because these techniques must be applied at the receiver, i.e. the user terminal.
At a user terminal with a single antenna, interference cancellation techniques that process multiuser signals with different signatures can be employed. Examples of these multiuser processors are given in Linear Multiuser Detectors for Synchronous Code-Division Multiple Access Channels, R. Lupas and S. Verdu, IEEE Transactions on Information Theory, vol. IT-35, No. 1, pp. 123–136, January 1989; Decorrelating Decision-Feedback Multiuser Detector for Synchronous Code-Division Multiple Access Channels, A. Duel-Hallen, IEEE Transactions on Communications, vol. COM-41, No. 2, pp. 285–290, February 1993; and, A Family of Multiuser Decision Feedback Detectors for Asynchronous Code-Division Multiple Access Channels, A. Duel-Hallen, IEEE Transactions on Communications, vol.COM-43, Nos. 2, 3, 4, February–April 1995.
Further, Transmitter Precoding in Synchronous Multiuser Communications, B. R. Vojcic and Won Mee Jang, IEEE Transactions on Communications, vol. 46, No. 10, October 1998, shows a precoding method employed at a single antenna transmitter to provide interference cancellation between quasi-orthogonal signals that have different signatures.
However, in an OMA technique the same channels or signatures are reused in adjacent beam coverage areas so as to increase the reuse factor. At a single antenna user terminal, there is no antenna discrimination; and, because the signatures are the same, there is no signature discrimination. Thus, the multiuser processor and transmitting precoding techniques referenced above are not applicable to a downlink OMA system with a single user terminal antenna.
Precoding at the transmitter in a downlink system is analogous to equalization at the receiver in an uplink system. Numerous algorithms for precoding, i.e., beamforming, have been proposed to reduce both co-channel, i.e., other user interference, and intersymbol interference in downlink transmissions. An example of such an algorithm is given in Transmit Beamforming and Power Control for Cellular Wireless Systems, F. Rashid-Farrokki, K. J. Ray Lui, and L. Tasseulas, IEEE Journal on Sel. Areas of communication, vol. 16, No. 8, pp. 1437–1450, October 1998. In this article a transmitter precoding method is described for cellular systems that reduces both other user and intersymbol interference. The objective according to the authors is to either reduce the frequency reuse distance or increase the channel capacity. However, the authors do not disclose that a reuse factor of unity, i.e., a frequency reuse distance of zero, can be achieved nor do they introduce and combine additional elements such as error-correction coding, interleaving, and periodic channel assignment changes.
Future multiple access radio systems will be unsymmetrical with typically greater downlink than uplink channel capacity requirements in order to satisfy Internet downloading demands. This future unsymmetrical capacity requirement places increased emphasis on finding techniques to increase downlink capacity.
The capacity of a downlink system is either limited by user interference in an OMA system, which keeps the reuse factor less than unity, or theoretically limited by the choice of QOMA. It would be desirable to have a multiple access scheme that can be used to obtain a unity reuse factor in downlink transmissions from one or more central nodes to a plurality of user terminals.